I. Field of the Invention
The present invention relates generally to a device for use in mobile recreational vehicles, marine vessels, and other such mobile self-contained living environments. The device monitors and computes the stored energy in the battery systems of such vehicles or vessels.
II. Description of the Prior Art
Self contained Recreational Vehicles (RV's) and marine vessels and the like typically contain on board battery systems, typically 12 volts DC to provide electricity to the living environments or cabins of such vehicles. The battery system consists of one or more batteries wired together in parallel and is normally electrically isolated from the vehicle prime mover battery which provides electrical power to start and run the engine or vehicle propulsion. In this manner, operation of the lights and appliances in the cabin will not discharge the prime mover battery. The cabin battery system is typically recharged by one of three different methods:
Typically when the prime mover battery is being charged, such as from an alternator being driven by an internal combustion engine, the two distinct battery systems are then connected together to permit charging the cabin battery system while the vehicle engine is running.
Another typical source of charging current to the battery is a solar panel, normally mounted on the roof of the RV. The solar panel generally consists of arrays of photo voltaic cells that convert sunlight into electricity and can therefore provide varying degrees of recharging current under varied weather conditions.
Yet another source of recharging current to the cabin battery systems are AC to DC converters which convert externally supplied 120 volt AC current when available from an RV park or dockside to the 12 volt DC level of the battery and regulate the charging rate. Commercially available units are very common which provide not only the electrical conversion and charging, but also provide current distribution and safety features. Terminals protected by circuit breakers are typically provided to which 120 volt appliances can be connected. Also typical is a distribution panel of several fused circuits to which the various 12 volt appliances can be connected.
As is known in the art, cabin battery systems for RV and marine use have different uses and requirements than the engine or starting batteries of vehicle propulsion systems. The engine battery is normally designed to provide a relatively large amount of current over a short time to drive a starter motor and start the engine. Although this requires large current flow, after the engine is running the battery is normally not required to provide more energy since electrical power is available from the engine alternator for vehicle loads such as headlights and the like and also for immediately recharging the battery itself. RV batteries, on the other hand, are normally required to provide relatively small to moderate current flow to operate the on board appliances such as 12 volt lights, water pump, heating furnace, and so on. Although the current demands are not large, they must be provided over very long periods of time, for example the many hours of a weekend or week-long camping trip. The Rv battery systems then are typically drained much closer to total discharge, then recharged by some combination of the charging methods previously discussed. These batteries are designed for much longer and frequent discharging-charging cycles than a vehicle starting battery and are referred to in the art as deep cycle batteries.
As is known in the art, at any point in time it is difficult to know how much energy remains in the battery system. The stored energy in the battery can be depleted slowly in powering such devices as lights, idle standby current draws of appliances like gas furnaces and water heaters, or small venting fans which draw little current. Although the current drawn is very small, typically a fraction of an ampere per device, the current drain is constant and when accumulated over time the energy depletion becomes appreciable. Conversely, appliances such as demand water pumps, heating furnaces or an inverter powering a microwave oven draw relatively large currents when active, typically 5 to 10 amperes per device. As is known, although the current drawn is large, the appliance use is of an intermittent nature as opposed to the constant but small current drains discussed. It is extremely difficult to ascertain at any point in time how much energy has been depleted due to the fact that a mixture of such slow and rapid depletion occurs, depending on which appliances are used for what intervals of time. Even if all durations of use for each appliance was monitored, the current drawn by each would need to be known, and in the prior art current indication is not often provided. Even if it were, the instantaneous current drawn is not a measure of the past history of energy depletion. In the prior art, commercially available panels are typically installed in Rv's which provide indication of the levels of fresh water supply and waste fluids in various tanks, as well as providing an indication for the battery. Unfortunately this indication is typically only that of battery voltage and is indicated at a poor resolution: empty, one-fourth, one-half, three-fourths, and full, sharing with the tanks this typically used array of 5 such labeled indicating lamps or LED's. Further, the battery voltage level measured and shown is instantaneous and therefore has no information regarding the energy depletion in past history, whether of an intermittent or constant nature or some combination of both.
Numerous examples in the prior art exist for estimating the state of charge of a battery, a notable example being a simple commercially device known as a hydrometer which with manual operation measures the specific gravity of the electrolytic fluid in a wet battery.
Regarding energy as opposed to state of charge, there is a deficiency in the art regarding measuring the actual energy used or depleted from the battery system over a given interval of time. In the field of battery systems of Rv's and the like, the given interval of concern might be that since a full overnight recharge. The energy depleted from the RV battery system is a function of the total power drawn by each appliance accumulated or integrated over the interval of time. There is a deficiency in the art since the instantaneous measurements of either current or voltage by their nature do not accumulate or integrate power draws over these intervals.
U.S. Pat. No. 4,958,127 of Williams et al derives the state of charge of a battery by measuring terminal voltage after application of a stabilization load. Like all other similar instantaneous state of charge devices, past history of energy drawn and energy capacities are neither monitored nor addressed.
U.S. Pat. No. 4,625,175 of Smith is an example in the prior art of such a device which measures the instantaneous open circuit voltage. Smith attempts to infer how much time remains in the life of the battery by providing a voltmeter with a scale from 11.7 to 12.7 volts and labeling the scale in increments of time. It is known that voltage potential expressed in volts and energy expressed in ampere-hours are fundamentally not equivalent. Therefore the assigned and labeled upper and lower limits, in addition to being arbitrary cannot be correlated to stored energy. U.S. Pat. No. 4,952,862 of Biagetti similarly attempts to predict the available reserve time remaining to a lower limit end voltage. Further, the time remaining from any point forward could not be known unless the current drawn from the battery was known ahead of time, such as a known constant. This is not the case in deep cycle RV batteries and the like since different appliances which have very different current demands are run in an intermittent manner.
The net energy remaining in an RV battery system at a point in time is due not only to the energy depleted from it since it was fully recharged, but also due to any partial charging which may have occurred during that time by any combination of the three charging methods described previously. For example, partial charging during sunlight by solar panels, and/or charging by the vehicle alternator during various travel segments, and/or partial night's stay at an RV park hookup for rest all may provide variable charge currents during a trip.
U.S. Pat. No. 5,032,825 of Kuznicki measures the battery voltage at two different current discharge rates and from the resultant differential attempts to infer the battery capacity. U.S. Pat. No. 4,849,700 of Moriok a attempts to detect low residual capacity of a battery by measuring battery voltage, and converting other instantaneous measurements into a conversion voltage value. These and many similar devices do not provide for the existence of any charging currents as would be present in a deep cycle or RV battery system. Further, as previously discussed, the voltage potential expressed in volts and energy capacity expressed in ampere-hours are fundamentally not equivalent and so the inference cannot be accurate.
Many other similarly questionable inferences and correlations exist in the prior art. U.S. Pat. No. 5,126,675 of Yang, for example, indirectly determines the equivalent internal resistance and attempts to correlate it to the remaining battery capacity. The resistance expressed in ohms and energy capacity expressed in ampere-hours are fundamentally not equivalent and so the inference cannot be accurate. Although simple and inexpensive in nature, and possibly correlatable in very special circumstances they cannot be accurate in the demanding applications inherent in the general operation of deep cycle batteries without knowledge of the past history.
It is known in the art that the deep cycle batteries utilized in Rv's and the like have effectively lower energy capacity at higher current draws. For example, a battery rated at 100 amp hours will be able to provide 5 amperes of current for 20 hours, but may only supply 10 amperes for 8 hours for a total of 80 amp-hours. Commercially available batteries in the recent art for this reason often carry ratings at two different current draws (e.g., in the present example 100 amp-hours at 5 amps and 80 amp-hours at 10 amps). This effective decrease in battery capacity means that to determine the energy remaining after some depletion a simple integration of current alone over time is not accurate. In particular, the owner or operator of an RV or the like could find the battery system dead while such a device inaccurately indicates some remaining stored energy. U.S. Pat. Nos. 4,051,424 of Privee and 4,678,999 of Schneider are examples of such integrating devices providing a digital amp-hour monitor. U.S. Pat. No. 4,740,754 of Finger and U.S Pat. No. 3,971,980 of Jungfer et al are examples of many other such devices incorporating similar integrating schemes.
A further deficiency in the prior art is apparent when also considering that the net energy level remaining in an RV battery system at any point in time is due not only to both the energy depleted and the energy restored due to recharging during some interval of time, but also the total energy that was present at the start of that time interval. Thus if the time interval is considered to be the time since a full recharge, the total initial energy is the battery capacity. However, as is known in the art, this energy capacity, typically advertised for a given battery in amp-hours, cannot accurately assumed to be constant since it decreases with time due to degradation of the battery itself. Further, the capacity of a given Rv battery system may also increase due to replacement of one or more batteries or augmentation with more batteries. Further, the actual capacity achieved by a battery in warm weather use may be different than that in cold weather. Therefore variations in capacity can occur due to seasons. Further, changes in regional climate from traveling can occur even during a given trip in an Rv: for example traveling in the Rv from desert to mountain regions or northern to southern regions.
It is apparent that there is a need in the art for a system which provides accurate information to the operator of an RV or marine vessel as to how much stored energy presently exists in the battery system.